Reciprocating drive mechanisms such as injection pumps and pneumatic pumps may be used to transport fluids, liquids, and/or gases and are generally used in various applications, such as manufacturing process control, hydraulic systems, and the like. The control valve of a reciprocating drive mechanism may comprise a spool valve coupled to piston. As a spool shifts between a first position and second position, control fluid may be selectively communicated between various ports and lines for pressurized movement of the piston. This pressurized movement preferably creates a pumping action of the piston through a repetitive series of motions.
U.S. Pat. No. 4,776,773, issued to Anthony J. Quartana, III (“Quartana”), for example, discloses a pilot control valve for changing the directional flow of fluid to a piston. The Quartana reference discloses a valve member or spool disposed within the upper housing. The spool is coupled to a piston via a valve stem and includes slide valves that are loosely mounted on the sides of the spool. The slide valves may shift between a first and second position on the spool and are used for selective communication of the control fluid. For example, when the slide valves are in the first position, the slide valves allow communication of the control fluid from the upper housing to the lower surface of the piston. As the slide valves move to their second position, they allow communication of the pressurized control fluid to the upper surface of the piston, thereby causing the piston to return to its first position. The Quartana control valve also includes an exhaust port located on the side of the housing for the release of fluid or air buildup behind the spool.
Unfortunately, these reciprocating pumps, especially the Quartana control valve, generally have a shorter spool design, which limits the stroke of the piston and spool to approximately 1-2 inches. Thus, these reciprocating drive mechanisms are not designed with the longer strokes because air buildup generally occurs behind the spool, thereby causing the pump to stall. Specifically, with each iteration of motion by the piston and spool, air is inadvertently forced behind the area of the spool due to the longer strokes. This air may be trapped and unable to escape, leading to air buildup, which can interfere with the operation of the pump. The trapped air may cause the pump to stall, possibly causing the pump to become inoperable and unreliable.
Therefore, there is a need for a new and improved reciprocating drive mechanism capable of performing longer strokes (e.g., at least four inches) while preventing air buildup behind the spool.